Field of Invention
This invention relates generally to integrators for totalizing with respect to time a signal whose level represents a process variable being measured to provide a total reading for a given period, and more particularly to a digitally settable drop-out for an integrator.
As explained in the article entitled "Signal Integrators" which appeared in the April 1966 issue of Control Engineering (page 76 et seq.), integrators may be used for totalizing an input signal representing any variable being measured. For purposes of illustration, however, we shall only consider integrators operating in conjunction with flow rate meters.
The present invention is addressed to problems arising in "drop-out" expedients presently included in such integrators. When a process is actually at zero, there is often some residual error signal from the process transmitter to the integrator. Thus some flowmeters have a square law function, which results in very large error signals under minimum flow conditions. Moreover, a small zero shift can also simulate a flow signal even though flow has ceased altogether.
Drop-out is the term generally used in the industrial process control industry to define expedients functioning to abruptly reduce a signal to zero when the signal falls below a predetermined level. A drop-out expedient in an integrator therefore serves to inhibit totalization when the input signal is at a level where it does not truly reflect the variable being measured, but represents a spurious value which, if added to the totalizer, would produce a misleading reading.
Since known forms of integrators are of the mechanical, electronic and pneumatic type, we shall consider typical integrators in each of these classes as well as the existing expedients incorporated therein to afford a drop-out action.
The mechanical integrator manufactured and sold by Fischer & Porter of Warminster, Pa. (F&P) and described in their Instruction Bulletin 52 T-4000B (August 1972) is included as a component in a flow rate measuring instrument to show the total cumulative quantity of the flowing fluid that is being metered by the instrument. This mechanical integrator receives its input signal via a connecting link from the instrument spindle that is indicating the process. The link acts to position a cam whose profile is shaped to represent the flowmeter curve.
A synchronous motor is used to operate the integrator, the motor driving a drive wheel with a pin mounted off center. As the pin moves through its orbit, it drives a spring-loaded follower arm, the follower arm causing a timed oscillation of the main shaft. A contact arm attached to the opposite end of the main shaft detects the profile of the cam representing the process.
If, therefore, the process variable is at 100% scale, the contact arm will oscillate through a 30% arc. At lesser process variable values, the contact arm travel is limited by the higher cam profile. Thus for each position of the cam, there is a discrete angle of rotation produced on the main shaft. Summation of the main shaft rotation is effected by means of a unidirectional clutch attached thereto. Actuation of this clutch engages a digital counter or odometer in the return stroke of the main shaft of the counter providing the desired total.
In the F&P mechanical integrator, the cam is profiled so that there is a step cut therein at the largest cam diameter. This is the zero input portion of the cam, and the step, therefore, functions as the drop-out expedient for the integrator.
The drawback of a mechanical integrator of this type, apart from its inherent mechanical limitations and complexity, is that the drop-out point is fixed and cannot be adjusted to meet varying situations which may dictate different drop-out settings to inhibit spurious signals.
In the pneumatic integrator manufactured by the Hokushin Electric Company of Japan and described in their Instruction Manual (August 1972) for Model ALI 101/102 (Bulletin No. E642-101), an incoming pneumatic signal representing flow rate is converted into a proportional lever displacement through a bellows-spring mechanism. The lever is U-shaped and carries on one of its arms a light-emitting diode and on the other, a photo sensor for picking up the light beam emitted by the diode. The diode is excited by electrical pulses at a constant repetition rate (i.e., 120 Hz).
A profiled vane rotated by a synchronous motor is interposed between the diode and the photo sensor to interrupt the light beam. The profile of the vane is such that the number of light pulses counted per single cycle of vane rotation is proportional to the input signal or its square root value. A frequency divider coupled to the output of the photo sensor reduces the counting rate of the pulses applied to an electronic counter which totalizes the count.
In the Hokushin integrator, in order to prevent erroneous counting at low flow rates (at less than 10% of full scale for linear signals, and less than 9% for square root signals), the vane has a drop-out step machined therein. Here again, the dropout is fixed; and while this is a low-cost approach, it makes no provision at all for situations requiring different drop-out settings.
An adjustable drop-out is included in the electronic integrator manufactured by F&P and disclosed in their Instruction Bulletin 52 ET 3000 (July 1973). This electronic integrator acts to totalize an electronic signal (i.e., 4-20 mA) with respect to time and to read the total for any given period. Operation of this electronic integrator is based on the integrating capability of an operational amplifier and utilizes a field effect transistor input amplifier as the active element. In order to keep the differential voltage at zero, the amplifier must force a charging current into a feedback capacitor that is equal to the current through the input resistance. Thus the rate of change of the output voltage (capacitor voltage) is proportional to the input voltage. Since this output voltage varies at a rate proportional to input voltage, the output is proportional to the integral of the input.
This electronic integrator includes a drop-out circuit to inhibit output counts whenever the count rate falls below a level selected by a drop-out potentiometer in an RC type of timer operating in conjunction with a frequency detector which senses the time interval between pulses by charging and discharging a capacitor once each flip-flop cycle.
If the time between pulses exceeds the period determined by the RC time constant, a reset pulse is applied to the scaler registers. While this type of drop-out is settable, it is subject to a large margin of error. For example, assuming a maximum divide in the scaling register and that a true count of 255 has been reached just when a drop-out occurs, then the scaler register will be automatically reset and the accumulated count of 255 will be erased from the register. Moreover, the RC time constant is influenced by changes in temperature, thereby introducing a further error-producing factor.